This proposal will focus on defining more precisely the role of complement activation products in acute inflammatory injury occurring after intrapulmonary deposition of IgG immune complexes in rodents (rats, mice). In the first aim, using mutants, (C3-/-, C3aR-/- and C5a-/-) or wild-type mice and blocking antibodies to mouse C3a or C5a, we will define the requirements for C3, C5, and the roles of C3a and C5a in development of acute lung injury. In the second aim, we will use C6-/- rats and determine if C6 (and by inference C5b-9) is required for the full development of acute lung injury. In the third aim we will pursue preliminary evidence that activated rat macrophages can, in the presence of C5, generate C5a, implying an extravascular source for generation of C5a. We will determine the nature of the C5 cleaving enzyme involved in this process. In the fourth aim we will use endothelial cells from different tissue/organ sources in rats and determine if, depending on the source of the endothelial cells, binding of C5a is similar or variable. We will also determine if functional responses to C5a (expression of P-selection) varies with the source of the endothelial cells. In the fifth aim we will stimulate rat lung microvascular lung endothelial cells with C5a or C5b-9 and, using microarray analysis, evaluate gene activation responses of the endothelial cells and compare these responses to TNFalpha. As time permits, these responses will be compared to those of rat dermal microvascular endothelial cells stimulated in a similar manner, since we now know that these two sources of endothelial cells exhibit very different functional behavior (susceptibility to killing by neutrophils). Finally, in the sixth aim, we will do microarray analysis of mRNA from whole rat lungs and from alveolar macrophages and BAL neutrophils in order to compare and contrast gene activation patterns in three complement-dependent models of lung injury in rats (IgG or IgA immune complex deposition and ischemia/reperfusion injury). These studies should extend our understanding of the roles of complement activation products in acute lung injury.